Fruit Burn: Extreme Heat or Irrigation & Soil Management Failure? [How can Evapotranspiration (ET) forecasting help?]

Hot weather causes fruit damage. Myth or fact?

OK, let’s dispel a big myth about hot weather and fruit damage.  Hot weather doesn’t automatically lead to crop damage.  It’s not a foregone conclusion – even in stone fruit.  Plants are nothing like humans so we shouldn’t expect that a plant will experience the same automatic displeasure because of “extreme heat” that humans do.  Many plants in my vegi garden and many commercial production crops I’ve visited in this weather are actually looking really healthy.  Good plant health in hot weather is a direct product of good growing conditions – temperature is only one thing affecting plant health and fruit quality.  Good growing conditions can certainly be influenced by good management.  The Riverland is a fantastic place to invest in growing great produce with more sunshine hours than the so-called Sunshine State.  Our hot dry climate often provides first access to seasonal markets and an edge over export competitors in other places.  Relative isolation and hot dry conditions also prevent some pests and disease from presenting in the Riverland.

So what causes fruit damage in hot weather and how can this be prevented or managed?

Put simply, fruit damage occurs when the plant transpires beyond being able to maintain firmness in the fruit it’s carrying.  This could be called water stress.  Provision and maintenance of adequate water is the core of this issue but there’s more to understanding what leads to water stress.

Soil scientists talk about Readily Available Water (RAW) within a soil profile.  RAW describes the potential capacity for storing water in a plants’ root zone that it can draw upon without experiencing stress.  If all RAW is used up by a plant, it will stress, and crop damage is a likely outcome.

Soils are mostly made up of sand, silt and clay particles that we call ‘texture’.  It’s useful to think of soil as a kind of sponge.  Some sponges have big holes with low density.  They might be good for a scrub in the tub, but they don’t hold a lot of water and dry out quickly.  Considering soil texture, these sponges would mostly be made of sand particles.  Conversely, some sponges are really fine and dense.  You can pour a fair amount of water into them without any water draining out.  If you wring them out as tight as you can, they still feel a little damp indicating some liquid remains trapped within.  Considering soil, this sponge is like a clay.  Clay soils can hold a lot of water but much of it cannot be accessed by plants.  A loam soil is a mixture of sand and clay and offers the most storage that remains accessible to plants.  Key point: soil texture impacts water storage available to a given crop and soil awareness needs management consideration – especially in hot weather.

Drip irrigation is widely considered best management practice in commercial horticulture and is now by far the most common form of irrigation system found in the Riverland.  Drip is the most efficient way to deliver water and fertilizer precisely where the plant needs it.  Drip minimises evaporation losses because it only wets up the area immediately around the crop.  The downside of drip irrigation is that by only wetting part of the whole orchard, plant roots can only utilise a proportion of the soil RAW capacity – typically about 1/3.  This means that growers need to manage irrigation more carefully as there is less water accessible for plants between irrigations.  This means applying less water more frequently.

Irrigation system maintenance:  A good irrigation design will have delivery capability to exceed crop water demand even during the hottest periods.  But even the best system needs regular maintenance and routine checks to ensure water delivery continues to occur evenly and efficiently.

Other factors like fertilizer program and applications, canopy management, crop variety, root stock, drip versus sprinkler challenges, soil salinity, irrigation system design and irrigation timing during the day can all have an impact beyond detail I can cover in this article.

So, what can be done to prevent crop damage in hot weather?

• Understand your data. When I say data, I’m talking about dig sticks and pressure gauges as a starting point.  Those simple devices can provide a lot of important information.  Of course, there are heaps of more sophisticated technology out there to assist but they are only as good as the quality and quantity of decisions they enable.
• Know your soils, their limitations and variation within management areas.
• Check that your irrigation system is operating correctly and at optimal efficiency. Is your system up to the challenge?  If not, what is the cost of not addressing the issues?
• Schedule irrigations based on soil storage capability and weather conditions. With today’s technology, you can be confident and plan ahead.  SWAN Systems uses the best 7 day ET₀ forecast in Australia produced by the Bureau of Meteorology.  For anyone interested, I can arrange a free daily weather forecast email including evapotranspiration (ET₀), specific for your location.
• Monitor what’s happening in soils. How far is water moving laterally?  Is irrigation penetrating beyond the crop rootzone and being wasted?
• Be really careful with fertilizer applications – avoid applications in hot weather.
• Plan for the next season ahead. This issue isn’t going away so learn from mistakes.  Identify system weaknesses and start thinking about how your management can improve.  By planning in advance, you can implement strategies for crops to better cope (or thrive) in hot weather.  Nutrient management programs can be used to target early leaf development with shading benefits and enhanced fruit quality.
• Get help if you need it – even in the heat there are still things you can do to make a difference. Identify the area(s) where you need help and seek specialist advice.

To Summarise…

Riverland soils are highly variable and there are many challenges for growers to overcome in growing an optimal crop.  Fruit damage in hot weather may not be preventable in some circumstances despite the best efforts of very competent growers.  This being said, the harsh reality is that uncontrolled crop stress occurs because a system has failed which means management of that system failed at some point too.  The system that failed might be inadequate or no soil survey back when the patch was first planted.  It might be that you are the beneficiary of an inadequate irrigation system that isn’t capable of delivering water in peak season (or not evenly).

Crop damage is not an automatic outcome of hot weather.  The Riverland is a great place to grow things even in the current climate!